1. Field of the Invention
This invention relates to a method of manufacturing a coated paper for printing. More particularly, the present invention relates to a method of manufacturing a coated paper for printing and having a coating face giving a superior suitability for printing.
2. Description of the Related Art
Japanese Examined Patent Publications (Kokoku) No. 49-24133 and No. 49-35447 disclose a curtain type coating method comprising a step of forming a free-falling curtain of a coating liquid, and a step of making the free-falling curtain collide with a supporting body, i.e., a coating base paper, to thus coat the paper, and it is well known that this method is suitable for providing a very thin thickness of the coating liquid and a very high processing speed.
A blade type coating method, a bar type coating method and a reverse-roll type coating method currently are mainly used as a method of manufacturing a common coated paper to be used for printing. In the two former methods, an excess quantity of the coating liquid is applied on the coating base paper and then the excess coating liquid is scraped off by a blade or a bar to obtain a desirable quantity of the coating liquid. In the reverse-roll type coating method, a coating liquid picked up by a coating roll is metered by a metering roll to obtain a desirable quantity of the coating liquid, and then the desirable quantity of the coating liquid is transferred to a coating base paper.
Nevertheless, an improvement of the quality of the coated paper for printing is urgently required, and thus the following problems with the above conventional coating methods have arisen.
Namely, a streak is generated on the coated paper due to an abrasion or a staining of a blade in the blade type coating method, which lowers the quality of the coated paper. Also, since the blade must be often replaced in this method, the operational efficiency of this method is poor. Also, the coating quantity is changed according to an irregularity of a surface of the coating base paper, and thus an unevenly coated surface often occurs. Accordingly, it is difficult to obtain a superior printing face of the coated paper. Further, this method has a drawback in that, when operational conditions are not suitable, the blade scrapes the surface of the coating base paper and the coating liquid is removed from the coating base paper in such a manner that fibers constituting the coating base paper are exposed at convex portions thereof. A composition of the coating liquid used in the blade type coating method must be kept at a level at which a viscosity of the coating liquid under the blade and subjected to shearing does not reach too high a value, to prevent generation of streaks, and thus a specific limitation of the characteristics of a pigment, an adhesive or the like, and a blending condition thereof, is required, and thus it is difficult to improve the quality, or lower the cost of manufacturing, of the coated paper.
Next, the bar (rod) type coating method has a limitation of a viscosity of the coating liquid used, and accordingly, it is extremely difficult to uniformly coat a coating liquid having a high viscosity on a coating base paper without a generation of coating irregularities. Further, this method has a drawback in that a quantity of the coating liquid to be coated depends largely on the viscosity and a concentration thereof, and thus it is impossible to freely change the quantity to be coated.
In the reverse-roll type coating method, a ratio between a surface speed of a coating roll and a surface speed of a metering roll must be successively changed according to the characteristics of a coating liquid, to thus obtain a desirable quantity of the coating liquid, and this procedure is very complicated. A nip space between the coating roll and the metering roll must be kept within an extremely narrow range, such as 0.025 mm to 0.62 mm, with a high precision, and thus a complicated apparatus and operation are required. Further, it is extremely difficult to obtain a coating surface having no irregularities by this method, and thus there is a limitation of a viscosity and a concentration of the coating liquid capable of be used in this method. Furthermore, when a solid foreign material is mixed in the coating liquid, the foreign material is apt to clog the space between the coating roll and the metering roll, and thus portions to which the coating liquid is not transferred are generated.
A curtain type coating method has been proposed for utilization in various applications. Namely, Japanese Unexamined Patent Publication (Kokai) No. 54-85811 discloses a utilization thereof in the manufacture of a pressure-sensitive copy paper, and Japanese Unexamined Patent Publication (Kokai) No. 54-74761 discloses the utilization thereof in the manufacture of a thermosensitive copy paper. Further, the curtain type coating method has been used in a process of wax coating a paper board. Nevertheless, the blade type coating method, the bar type coating method or the reverse roll type coating method is now commonly used for the manufacturing of a coated paper.
A reason why the curtain type coating method cannot be commonly applied to the coated paper is that a concentration and a viscosity of the coating liquid used for a coating paper to be used in printing cannot be lowered, due to problems regarding a quality of a final product and a consumption of drying energy. For example, a viscosity of the coating liquid used for the manufacture of the pressure-sensitive copy paper and the thermalsensitive copy paper is between 100 cps and 400 cps, but a coating liquid having a viscosity of 1000 cps and 4000 cps is used for the manufacture of the usual coated paper, and therefore, if this coating liquid is directly used in the curtain type coating method for the usual coated paper, uncoated portions caused by bubbles in the coating liquid are generated on the coated face, and the thus-generated bubble traces are elongated by a difference between a coating speed and a falling speed of the coating liquid to a length several times the size of the bubbles in the coating liquid. The generation of such elongated traces is a very serious problem when a high speed coating process, such as a process used for manufacturing the coating paper to be used for printing is adopted.